1. Technical Field
The present invention relates to a method for monitoring a PSM machine, and an assembly for monitoring a PSM machine.
Permanently excited synchronous motors, also referred to as PSM machines, are used in a motor vehicle for different purposes, e.g. steering force support, a traction drive, or other drives. A PSM machine is an induction machine, with a permanent magnet disposed on or in the rotor. The at least one stator comprises windings of three or more phases, and forms phases therefrom, distributed at an angle of 120°. The coils of the phases are distributed on a circumference about a rotational axis, in relation to which the rotor is rotatably supported relative to the stator.
2. Background Information
The state variables of the PSM machine, such as current, voltage, flux, etc. can be depicted in a three-dimensional coordinate system (U, V, W), as is shown in FIG. 1. The state variables of the machine are transformed into a coordinate system (d, q) rotating with the rotor for the regulation of the PSM machine, wherein the d-axis has the same direction as the permanent flux of the rotor, as is likewise shown in FIG. 1. The α, β-system is the stationary two-coordinate system of the stator. By transforming the state variables into the d, q-coordinate system, the differential equations of the PSM machine are simplified, and the PSM machine can be regulated in the manner of a direct current machine. This is referred to as field oriented regulation, or FOR. With a field oriented regulation, an overall target current, which is to flow through the induction machine, is determined with respect to a rotor (flux) stationary d, q-coordinate system, such that some control or regulation processes are easier to execute, and some calculations are simplified.
Normally, electrical currents through the individual phases, usually interconnected in a star shape, are sampled. With this circuitry, the sum of the currents through the individual phases is always equal to zero. This means that the phase current through one of the phases can thus be determined from the sum of the phase currents in the other two phases. If, however, a current sensor for determining the phase current of one of the phases malfunctions, then the field oriented regulation can no longer be carried out normally with only one current sensor. With known methods, the measurement of currents is necessary for diagnosis. It may still be possible, however, to operate the machine.